This invention relates to a process for displacing oil within a permeable material such as a subterranean reservoir. More particularly, it relates to a process for displacing oil within a permeable material in which the oil is mixed with an aqueous solution containing multivalent cations.
It was previously known that aqueous anionic surfactant systems are particularly effective and efficient for displacing oil within permeable materials such as subterranean reservoirs. Such processes have been used for chemical flood-type oil recovery operations, for well treatments to displace residual oil from locations in which it impedes the permeability to aqueous fluid, and the like. Such aqueous anionic surfactant systems include surface active salts or soaps of organic or inorganic acids and the systems may be pre-formed or, at least to some extent formed within the material in which the oil is being displaced. As disclosed in the J. Reisberg U.S. Pat. No. 3,174,542, the acids that form surface active soaps can be injected ahead of an aqueous alkali that forms such a soap. Mixtures of relatively water-soluble and relatively water-insoluble petroleum sulfonates in an aqueous solution of electrolytes are described in the J. Reisberg U.S. Pat. Nos. 3,330,344 and 3,348,611. Aqueous liquid systems containing anionic surfactants electrolytes and water-thickening materials are described in the H. J. Hill -- D. R. Thigpen U.S. Pat. No. 3,768,560, etc.
Although aqueous anionic surfactant systems are generally particularly effective and efficient in displacing oil, they exhibit a relatively low tolerance towards aqueous solutions containing multivalent cations. Such cations tend to reduce or destroy the efficiency of the system by precipitating multivalent cation salts of the anions of the surfactant or inducing a phase separation of the surfactant system components, of the like. Such problems are discussed in patents such as the J. Reisberg, J. B. Lawson U.S. Pat. No. 3,508,612, the R. F. Farmer, J. B. Lawson, W. M. Sawyer, Jr. U.S. Pat. NO. 3,675,716, U.S. Pat. No. 3,799,264, etc. As indicated by such patents, prior processes for eliminating the deliterious effects of multivalent cations have involved mixing sulfonate surfactants with alkoxylated alcohol sulfate surfactants; or using aqueous surfactant systems containing a mixture of a sulfated aliphatic anionic surfactant, a non-ionic surfactant, and a water-soluble quanidine compound, or the like.